The invention relates to a multistation communication bus system accommodating master stations and slave stations, any master station comprising frame based arbitration means, arbitration outcome detection means, and frame formatting means for upon winning said arbitration transmitting a framewise organised message including locking and unlocking signalizations for locking and unlocking respectively, an addressed slave station for a duration of multiple frames contained in said message, so that a different master station is blocked from at least in a particular manner accessing a locked slave station during said duration. Generally, such communication systems impose a maximum value on the allowable length of a frame, so that a particular master station cannot keep the whole system engaged for more than a reasonable interval of time. The consequence of this is that if a master station wants to send a long message to a particular slave station, this must be done in a sequence of frames that collectively constitute the message. In such case, the master station in question should be enabled to keep an addressed slave station free from addressing by other master stations between successive frames of this message. For this object the locking mechanism has been proposed, such as described, for example, in U.S. Pat. Nos. 4,937,816; 5,128,936; 5,249,182 (PHN 12.484) assigned to the assignee of the present application and herein incorporated by reference. In a particular embodiment, the maximum locking period may be 300 milliseconds. Now, if another master station tries to send a frame to the slave station in question, this will not be successful. The solution may be that the second master station repeats its undertaking at a later time. Nevertheless, the access should be as quickly as reasonably possible, in situations of either a locked or an unlocked slave station. In this respect it should be clear that even in an unlocked situation, the accessing by the further master station may be temporarily blocked, for example because the slave station has its receive buffer still filled with a previously received message. Other, less frequent causes for such blocking are electrical interference and temporary error.